As a color still image encoding scheme, JPEG as the international standard is prevalent. As a method of realizing higher compression ratio than JPEG, a new encoding method based on wavelet transformation is being standardized.
As the processing speed of a CPU increases, a moving image can be encoded by continuously encoding using an image encoding apparatus that adopts the aforementioned encoding scheme. A moving image input scheme to the image encoding apparatus includes progressive scan for sequentially scanning an image in units of lines from the upper left corner toward the lower right corner of a frame, and interlaced scanning for scanning an image in two scans, i.e., odd and even lines by interlacing lines.
However, since the encoding scheme used in conventional still image encoding adopts progressive scanning, if a moving image input by interlaced scanning is directly applied to still image encoding, the coding efficiency drops considerably.
An example of such case will be described in detail below.
In interlaced scanning, the scan frequency is doubled while halving the number of pixels by scanning an image every other line. FIG. 29 is an explanatory view showing this timing. If the frame period of progressive scanning is {fraction (1/30)} sec, that of interlaced scanning is {fraction (1/60)} sec.
When a moving image input by interlaced scanning is processed as that in the progressive format, two field images are processed as one frame image. The number of pixels processed per frame period is the same in both the scanning schemes.
FIGS. 30A to 30C are views for explaining the state wherein an image in which a vertical line with a given width moves from the right to the left on the screen is captured.
FIG. 30A shows an image captured at the frame period. FIG. 30B shows images captured at the field period. FIG. 30C shows an image obtained by displaying the images shown in FIG. 30B at the frame period. Since the capture timings of the images shown in FIG. 30B have a time difference, the displayed image shown in FIG. 30C deviates in the horizontal direction. This deviation contains high-frequency components when viewed from the vertical direction. As a general feature of a natural image, many coefficients are contained in a low-frequency range when the image is broken up into subbands. For this reason, the low-frequency range is broken up into subbands again in FIG. 30C. However, an interlaced image also contains many components in a high-frequency range, as described in the example shown in FIGS. 30A to 30C.
When many coefficients which are naturally concentrated in LL appear in HL, or when an original signal contains high-frequency components in the horizontal direction, many coefficients appear in both LL and HL. Under the influence of interlaced scanning, more coefficients appear in LH and HH, thus considerably impairing the efficiency of subsequent entropy encoding.
When an image does not move in the horizontal direction within {fraction (1/60)} sec, since high correlation is found in the vertical direction, high encoding efficiency can be obtained by encoding in units of frames as in conventional still image encoding.
In order to efficiently encode an interlaced moving image, a process in units of frames and that in units of fields are preferably combined.